Abstract:
The Miocene-Pliocene Nagri Formation of the Siwalik Group is molasse deposit
exposed in the Defence housing Authority (DHA) Valley, Kallar Syedan, Rawalpindi.
Here the formation comprises sandstone facies (N1), and sandstone-siltstone-mudstone
facies (N2). The major lithology of the formation is green, fine- to medium -grained,
and thin- to thick sandstone beds with intercalation of siltstone and maroon mudstone.
Graded and cross-bedding, spheroidal and differential weathering in the sandstones are
the common features observed. In thin section study, the rocks contain monocrystalline
and polycrystalline quartz, alkali feldspar, plagioclase, rock fragments, biotite,
muscovite, zircon and epidote. Groundmass is calcite and at places mixed calcite and
clayey material. Alkali feldspar is microcline and perthite whereas anorthite content in
plagioclase ranges up to 10%. Both the feldspars are partly altered into sericite, calcite
and epidote. Rock fragments constitute volcanic and metasedimentary rocks. On the
basis of petrography, the sandstones of the Nagri Formation of the Siwalik Group fall
into feldspathic and feldspathic litharenites type domains. The dominance of alkali
feldspar over plagioclase suggests granitic composition of the eroded rocks, probably
derived from Lesser and Higher Himalayas and/or the Kohistan Island Arc. The
presence of mica, epidote, amphiboles and illite may suggest low-medium grade
metamorphism encountered in the area before the uplift and erosion. Q-F-RF ternary
plot distinguish the sandstone and siltstone of the Nagri Formation as recycled orogen
and dissected arc provenance. The medium to coarse-grained sandstone may signify it
as sandy bedload deposit due to river action. The intercalation of sandstone, siltstone
and mudstone may indicate, the deposition in a relatively calm depositional
environment.
